1. Field of the Invention
The present invention relates to a detection device, a developing device, and an image forming apparatus, and in particular relates to detection control for detecting toner charge amount.
2. Description of the Related Art
An electrophotographic image forming apparatus forms a toner image by forming an electrostatic latent image on a photoreceptor on the basis of a document read by a reader or an image transmitted from an external PC and developing the electrostatic latent image on the photoreceptor using toner in a developing unit. The image forming apparatus controls the density of the toner image by controlling image forming conditions such as the amount of exposure to a laser beam emitted from an exposure apparatus for forming the electrostatic latent image on the photoreceptor, a developing bias for developing the electrostatic latent image on the photoreceptor, and a charge potential for charging the photoreceptor. However, if the toner is consumed from the developing unit or toner is additionally supplied to the developing unit while a large number of toner images are being formed, the charge amount of the toner in the developing unit will change. Further, the charge amount of the toner in the developing unit changes also due to changes in temperature or humidity inside the developing unit. In order to control the density and color tone of the toner image with high accuracy, it is desired that the image forming conditions are controlled according to the charge amount of the toner in the developing unit.
An apparatus that includes a probe for collecting a very small amount of toner from a magnetic brush roller of a developing unit and is configured to measure the charge amount of toner in the developing unit on the basis of the mass of the toner collected by the probe and the change in the charge amount on the magnetic brush roller has been proposed (see the specification of U.S. Pat. No. 5,006,897).
According to the specification of U.S. Pat. No. 5,006,897, toner on the magnetic brush roller of the developing unit is first adsorbed by a probe provided with a piezoelectric crystal resonator and an electrode, and the piezoelectric crystal resonator is caused to vibrate. The mass M of the toner adhering to the probe is calculated based on the difference between a vibration frequency in the state where the toner is adhering to the probe and a vibration frequency in the state where no toner is adhering to the probe. Further, since the toner is moved from the magnetic brush roller to the probe, the charge amount Q of the toner adhering to the probe can be determined by measuring the change in the charge amount of the toner on the magnetic brush roller. The charge amount of toner in the developing unit can be detected on the basis of the mass M and the charge amount Q of the toner adhering to the probe.
However, it has been experimentally proven that the vibration frequency in the state where the toner is adhering to the probe is actually the same as the vibration frequency in the state where no toner is adhering to the probe, and thus the mass M of the toner adhering to the probe cannot be detected.
One possible reason for this is that the toner that is to be detected slides off due to the vibration of a QCM sensor. For example, the mass detection surface of the probe is subjected to shear vibration at a resonance frequency of several [MHZ] to 10 [MHz]. The vibration direction is a single direction parallel to the electrode surface, and the amplitude thereof is about several [nm]. In order to detect the mass of toner, the toner has to be held on the mass detection surface without sliding off due to the shear vibration of the mass detection surface.
The present invention provides a measuring device, a developing device, and an image forming apparatus, which are capable of detecting the toner charge amount with high accuracy by suppressing the sliding of toner during the vibration of a piezoelectric crystal resonator.